This plane left the SR-71 Blackbird in the dust

The SR-71 Blackbird was the fastest military jet that has ever taken to the skies. But there was a plane that not only went twice as fast, but it also went much higher.

That speedy plane was the North American X-15.

The X-15 was one of the first true spaceplanes, with a number of flights going beyond Earth's atmosphere, according to a 2005 NASA release. It was capable of going over 4,500 mph, or nearly Mach 6, and it went as high as 354,200 feet – or just over 67 miles – above the Earth.

North American X-15A. (NASA photo)

The plane didn't actually take off from the ground. In fact, it needed the help of a B-52 bomber before it could reach those dizzying heights and super-high speeds. NASA used two of the first B-52s, an NB-52A known as the "High and Mighty One," for some flights before a NB-52B known as "Balls 8" took over the duty.

Once released from the B-52 at an altitude of 45,000 feet and a speed of 500 miles per hour, the X-15's Reaction Motors XLR-99 would activate providing 70,400 pounds of thrust, according to a NASA fact sheet. At most, the plane had two minutes of fuel.

A X-15A with external fuel tanks and a new paint job is dropped from a NB-52 aircraft. (NASA photo)

Among the pilots who were at the controls of this marvel was Neil Armstrong – you'd know him as the first man to walk on the moon. Armstrong didn't get into space with this plane in any of his seven flights, but he did post the 6th-fastest speed among the X-15 sorties, according to an official NASA history.

One of those who achieved the rating of astronaut, Major Michael Adams, received the honor posthumously after he was killed in a crash of his X-15A on Nov. 15, 1967. Adams had broken the 50-mile barrier that the Air Force and NASA used to define entering space on his seventh and final flight, reaching an altitude of 266,000 feet and a top speed of 3,617 mph, according to the NASA history's list of X-15 flights.

Below, take a look at the video from Curious Droid, which talks about the X-15 – and the awesome career it had.